Hi and welcome to the forum. I am not familair with your coach or the Girard water heater specifically, but since no one has responded yet I thought I would start the ball rolling.
It sounds from your description that water is flowing through the hot water heater OK, but just not being heated? I also assume you have no winterization bypass valves on your coach? (Girard does not recommend using them because they can result in reduced flow into the water heater if not adjusted properly to summer position or if leaking etc).
Instant water heaters are sensitive to water flow/pressure at the inlet side and may not actually turn on if the flow/pressure is not high enough. I think Girard even states that it is better to not mix hot and cold water taps to get a desired sink/shower water temperature, but rather use the hot only tap and set the output temperature of the water heater to a desired heated water temperature. The reasoning is that opening the cold tap can reduce the flow/pressure on the inlet valve to the instant water heater under some camp ground/coach pump, water pressure conditions. I also think the Girard heater has an adjustable inlet control valve to allow more water into the unit which may better trigger the water heating cycle. There could be other issues causing no heat, but the Girard manual should suggest other items to check if its not the inlet water pressure.